1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a method of fabricating a CMOS image sensor. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for enhancing light-condensing efficiency by improving a curvature of a lens in a CMOS image sensor.
2. Discussion of the Related Art
An image sensor is a semiconductor device that converts an optical image to an electric signal. Image sensors can be classified into charge-coupled devices and complementary metal-oxide-semiconductor (CMOS) image sensor devices.
The CMOS image sensor includes a photodiode unit sensing an applied light and a CMOS logic circuit unit processing the sensed light into an electric signal as data. Photosensitivity of the image sensor is enhanced if a quantity of light received by the photodiode is raised. To enhance the photosensitivity, a fill factor (e.g., the photodiode area divided by the entire area of image sensor) may be raised, or a path of light incident on an area other than the photodiode may be diverted to condense or direct the diverted light to the photodiode.
For example, a microlens may be used in condensing the diverted light onto the photodiode. By providing a convex microlens formed from a material having good light transmittance over a photodiode, a path of an incident light may be refracted towards the photodiode. Hence, more light can be applied to the photodiode area. In doing so, a light parallel to an optical axis of the microlens can be refracted by the microlens to focus at a prescribed position on the optical axis.
Referring to FIG. 1, a CMOS image sensor according to a related art includes one or more photodiodes 11 formed on a semiconductor substrate (not shown) to generate electric charges according to a quantity of incident light received on the photodiode(s) 11, an insulating interlayer 12 formed over the substrate including the photodiodes 11, a protective and/or first planarizing layer 13 formed on the insulating interlayer 12, a R/G/B color filter layer 14 formed on the layer 13 to transmit light having a specific wavelength therethrough, a second planarizing layer 15 formed over the substrate including the color filter layer 14, and a microlens 16 formed on the second planarizing layer 15 having a convex shape with a predetermined curvature and adapted to condense the received light onto an area of the corresponding photodiode 11 through the color filter layer 14.
An optical shielding layer (not shown) may be provided within the insulating interlayer 12 to prevent the light from one filter in the color filter layer 14 from entering another photodiode area except the corresponding photodiode 11 (i.e., the photodiode directly below a given filter). The photodiode can be replaced by a photo gate to sense the light.
In this case, a curvature, height (A), and like dimensions of the microlens 16 are determined by considering various factors including a focus of the condensed light and the like. The microlens 16 is mainly formed from a polymer-based resin by deposition, patterning, reflowing and the like. Namely, the microlens 16 has a size, position and shape related to a corresponding unit pixel and a thickness of the photosensitive device thereunder, and generally has an optimal size determined according to a height, position, size and the like of the optical shielding layer, and the radius of curvature.
Meanwhile, the shape of the lens pattern profile depends on an exposure condition (e.g., focus) of the photoresist. For instance, one or more process conditions for making the microlens can be varied according to a thin film situation of a semiconductor substrate, whereby a profile of the microlens may be changed. Since the pattern forming condition can be relatively unstable, the light-condensing efficiency can be less than optimal. Thus, in the process for fabricating the related art CMOS image sensor, the microlens 16, provided to enhance a light-condensing power, is an important factor and/or structure affecting characteristics of the image sensor. A microlens having increased light-condensing efficiency transmits more light to the photodiode 11 through the corresponding color filter layer 14 (which selects a color for detection by photodiode according to the wavelength of light that passes through it). That is, the light incident on the image sensor is condensed by the microlens 16 and filtered by the color filter layer 14, and then it enters the photodiode 11 under the color filter layer 14. In doing so, the optical shielding layer (not shown) plays a role in preventing the incident light from deviating to another light path and entering a different photodiode (usually for a different color, and always corresponding to a different location in the CMOS image sensor array).
FIG. 2 shows a color filter layer on a protective and/or planarizing layer in a CMOS image sensor according to a related art. Here, a plurality of rectangular color filter layers 14 are arranged on the protective and/or planarizing layer 13, uniformly spaced apart from each other.
However, the convex microlens in the related art CMOS image sensor has the following problems.
First of all, in the case of hemispherical microlenses, the light parallel to the optical axis is refracted by the lens and then arrives at the photosensitive device, which is located at a position opposing the lens, to operate the device normally. Yet, light that is not parallel to the optical axis may be refracted by the lens and arrive at another photosensitive device that is not supposed to receive the light. Hence, the device may operate abnormally.
Secondly, since a quantity of light arriving at the photosensitive device varies according to the species and thickness of the layer under the microlens, the light-condensing efficiency may be reduced and degrade the quality of the image.